subject: Electronic ballast having adaptive frequency shifting [print this page] Electronic ballast having adaptive frequency shifting
Electronic ballast for fluorescent lamps typically can be analyzed as comprising a "front-end" and a "back-end". The front-end typically includes a rectifier for changing alternating-current (AC) mains line voltage to a direct-current (DC) bus voltage, and a filter circuit, e.g., a capacitor, for filtering the DC bus voltage. The front-end of electronic ballasts also often includes a boost converter, which is an active circuit for boosting the magnitude of the DC bus voltage above the peak of the line voltage and for improving the total harmonic distortion (THD) and the power factor of the input current to the ballast. The ballast back-end typically includes a switching inverter for converting the DC bus voltage to a high-frequency AC voltage, and a resonant tank circuit having a relatively high output impedance for coupling the high-frequency AC voltage to the lamp electrodes.
Referring first to FIG. 1, there is shown a simplified block diagram of a prior art electronic ballast 100. The ballast 100 includes a front-end 102 for producing a substantially DC bus voltage across a bus capacitor, CBUS, from an AC input voltage. The ballast 100 further comprises an inverter 104 for converting the DC bus voltage into a high-frequency voltage for driving a lamp current in a fluorescent lamp 108. The high-frequency voltage provided by the inverter 104 is coupled to the lamp 108 through a resonant tank 106 having a resonant inductor, LRES, and a resonant capacitor, CRES.
A simplified schematic diagram of another prior art electronic ballast 300 is shown in FIG. 2. The ballast 200 operates in a similar manner as the ballast 100 shown in FIG. 1, but the analog control circuit 210 has been replaced by a digital control circuit 310. An analog-to-digital converter (ADC) 352 in a microprocessor 350 receives the lamp current signal 250 from the current sense circuit 110 and converts it into an 8-bit digital representation. The reference signal 242 representative of the target current in the lamp 108 is received at an input 355. The software in the microprocessor 350 then compares the measured current with the target current to generate an error signal, which is then used to generate a desired duty cycle. The desired frequency is determined from the desired current. A pulse-width modulated (PWM) signal 356 is produced at an output 354 of the microprocessor 350. The software in the microprocessor 350 drives the PWM signal 356 with the desired frequency and duty cycle and provides the PWM signal to the gate drive circuit 116. In the ballast 300, software in the microprocessor 350 of the digital control circuit 310 provides the functionality that was provided by the analog control circuit 210 of the ballast 100.
According to the present invention, an electronic ballast for driving a gas discharge lamp includes an inverter, a resonant tank, a control circuit, and a current sense circuit. The inverter converts a substantially DC bus voltage to a high-frequency AC voltage having an operating frequency and an operating duty cycle. The resonant tank couples the high-frequency AC voltage to the lamp to generate a present lamp current through the lamp. The control circuit is operable to control the operating frequency and the operating duty cycle of the high-frequency AC voltage of the inverter. The current sense circuit provides to the control circuit a present lamp current signal representative of the present lamp current. The control circuit is operable to control the operating duty cycle of the high-frequency AC voltage of the inverter in response to a target lamp current signal and the present lamp current signal. Further, the control circuit is operable to control the operating frequency of the high-frequency AC voltage of the inverter in response to the operating duty cycle and a target duty cycle, such that the control circuit is operable to minimize the difference between the operating duty cycle and the target duty cycle. Preferably, the control circuit is further operable to control the operating frequency to a base operating frequency in dependence on the target lamp current signal, when the target lamp current changes in value.
The present invention further provides a method for controlling an electronic ballast for driving a gas discharge lamp. The ballast comprises an inverter characterized by an operating frequency and an operating duty cycle. The method comprises the steps of generating a present lamp current through the gas discharge lamp in response to the operating frequency and the operating duty cycle of the inverter; generating a present lamp current signal representative of the present lamp current; receiving a target lamp current signal representative of a target lamp current; controlling the duty cycle of the inverter in response to the target lamp current signal and the present lamp current signal; and controlling the operating frequency of the inverter in response to the target lamp current signal, the operating duty cycle of the inverter, and a target duty cycle, such that the difference between the operating duty cycle and the target duty cycle is minimized.
In addition, the present invention provides a control circuit for an electronic ballast having an inverter for driving a gas discharge lamp. The control circuit is operable to control an operating frequency and an operating duty cycle of the inverter of the ballast. The control circuit comprises a duty cycle control portion for controlling the operating duty cycle of the inverter in response to a target lamp current signal and a present lamp current signal, and a frequency control portion for controlling the operating frequency of the inverter in response to the target lamp current signal, the operating duty cycle, and a target duty cycle. The difference between the operating duty cycle and the target duty cycle is minimized.
